After having played around 1500 hours of AoW 3 and I dont know how many hours of Shadowmagic and AoW 1, I can see some things still missing. I would say that the same interface for the 3 would be great, since it was very easy (both for the casual player and the advanced) and quick to get used to (with planetfall there is a bit of a startup to learn how to find the info you need and that may discourage players not fans of the series. I would keep AoW III more or less the same (sure, hot up the graphics and the sound, but otherwise let it be) and only add things to an allready great game. Things I would add is the following:
1. Diplomatics: The Planetfall diplomatic screen is great with a lot more options.
2. Let the builder live a little: Allow it to actually do more stuff, like building small farms, lumbermills aso. Also maybe small trade huts for the new trade system (se below)
3. Let the sea be important: A great way to do this is with trade routes. In my wiev, a trade route in AoW 4 would do two things, either generate gold or transport special substances. The special substances allow you to build the units with special abilities (like for example, build infantry units with Fear strike) in a city that doesnt have the special dwelling in its area. This trade route system would make sea baing a lot more important, since the trade routes could be plundered and needs to be protected. A whole new tech could be built around this, starting with 2 unarmed caravan units and ending with 6 juggernauts carrying around your goods. Also all the sea areas should produce growth to the city and if there is an "Enemy in the domain", this growth is cancelled. This would make sea a valuable resource.
4. Allow all units that are no race specific to be able to "Evolve". (for example, Troll to troll King, feathered serpent to ancient feathered serpent aso).
5. Allow 2 different casting times of combat spells. One that doesnt en the turn for the hero (since in AoW III you hade several spells that while cool was not used since they took an important action from the hero that he could use in much better ways) and the other functioning at it does today.
6. Reintroduce some skills from Shadow magic, for example Round attack
7. Randomzied maps that allow you to easy build new maps. AoW III hade hell with this wich made mapmaking a drag. Allow a player to use a randomized map and correct it and presto you can put a new map in steam
8. Listen to the modding community. Do something like Xcom. The more they listened and allowed the modding community to put in inputs, the better the game became.
Well, that is about it.